The present invention relates to a point level detector to sense the level of a liquid. More particularly, the present invention relates to a point level detector adapted to sense the level of a liquid having a temperature above or below ambient temperature by sensing the increased heat transfer coefficient of the liquid.
The prior art has provided point level detectors for sensing a liquid level of a cryogenic liquid, such as liquid nitrogen, contained within a container. Point level detectors are so called because they serve to indicate whether liquid is above or below a predetermined level in a container. This is to be contrasted with continuous level detectors that measure the level of a liquid within the container.
Point level detectors of the prior art that are specifically designed to sense cryogenic liquid level often employ self heating temperature sensing elements. Such self heated temperature sensing elements generate a temperature signal related to absolute temperature. When the level of the liquid falls below the temperature sensor, the temperature sensing element self heats because the gaseous form of the cryogen conducts heat at lower rate than the liquid form of the cryogen. Such self heating of the sensing element is accompanied by an increase in temperature and a consequent change in the temperature signal to indicate the drop in the liquid level. An example of such a point level detector can be found in U.S. Pat. No. 3,755,801 which employs silicon diodes as the self heating temperature sensing elements.
There are point level detectors, such as disclosed in U.S. Pat. No. 3,938,347, that utilize temperature sensing elements, formed by thermocouple junctions, that in use do not appreciably self heat. In the '347 patent, a resistor is positioned close to a thermocouple junction to heat the junction. The submergence of the resistor and thermocouple junction within the liquid cryogen cools the resistor to decrease the magnitude of the signal generated by the thermocouple junction and thus supplies an indication of the level of the liquid cryogen.
As will be discussed, the present invention provides a point level detector that is less complicated than those of the prior art.